In such a disc player equipped with a focus servo control system of the above-mentioned type, a laser light beam is to be precisely converged through an object lens onto a recording surface of a recording disc (i.e. a recording medium) while the recording disc is rotated. The laser light reflected from the recording disc is converted into an electrical signal by means of a photoelectric converter included in the focus servo control system, the electrical signal representing the video and/or audio information stored in the recording disc. The video information is imaged on a monitor of the video disc player and the audio information is reproduced by a speaker system of the player, respectively.
Since a recording disc is subject to be warp and mounts in a slightly swerved manner on a turntable, an optical pickup system for use in a video disc player requires a focus servo device which controls the position of the object lens to precisely converge the laser light beam into a desired position with respect to the recording surface of the recording.
In a known focus servo device, the gain of a servo amplifier is fixed at a certain value. It is known that such a prior art focus servo device can not cope with relatively intense external disturbances, without the gain being fixed at a relatively large value. However, if the gain is at a large value, a correspondingly large amount of electrical power is consumed and also mechanical noises are developed due to a so-called piston motion of a system carrying the above-mentioned object lens in the focus servo device.